1. Field of the Invention
The present invention relates generally to an apparatus and method for recovering a phase of a pseudo-random noise (PN) sequence from a pilot signal by a mobile terminal in a mobile communication system, and for instance, to an apparatus and method for tracking a phase of a PN sequence, using a Tau-Dither Loop (TDL) scheme.
2. Description of the Related Art
In a mobile communication system, a pilot signal provides a mobile terminal with timing information, phase reference information and reference information for identifying a base station. The pilot signal is always assigned a Walsh code #0, and spread by a PN sequence, with no information carried thereon. That is, the pilot signal becomes a PN sequence.
The pilot signal provides information on a continuous basis or on a burst basis. While a conventional voice-centered mobile communication system has provided a continuous pilot signal, a packet-centered multimedia system such as 1× EVolution-Data Only (1×EV-DO), Telecommunications Industry Association/Electronic Industries Alliance/Interim Standard-856 (TIA/EIA/IS-856), system provides a burst pilot signal in order to optimize transmission efficiency of the system.
FIG. 1 is a block diagram illustrating an example of a frame format for a mobile communication system in which a burst pilot signal is provided. As illustrated, for N1-chip durations 101 and 105, a pilot signal is periodically transmitted. In addition, for N2-chip durations 103 and 107, information data is periodically transmitted. The burst signal represents signal transmitted periodically or signal transmitted by a predetermined time. The present invention touchs on pilot signal as a burst signal. In the 1×EV-DO system, N1 and N2 means 96 chips and 928 chips, respectively.
In a receiver, or a mobile terminal, a process of recovering a phase of a PN sequence from the pilot signal is roughly divided into an acquisition step and a tracking step. In the light of a phase recovery resolution, the PN sequence acquisition step refers to a process of adjusting a phase of a local PN sequence so that a difference between a phase of a PN sequence of a received pilot signal and a phase of the local PN sequence generated in the receiver falls within a 1-chip length. Generally, the PN sequence acquisition step is performed in a searcher.
The PN sequence tracking step is to precisely track a phase of a PN sequence acquired by the searcher so that a phase difference between the acquired PN sequence and the PN sequence of the received pilot signal becomes less than a 1-chip length. In addition, the purpose of the PN sequence tracking step is to prevent reception quality deterioration due to relative movement of a base station and a mobile terminal or unstableness of a reference clock within the receiver. In a receiver of a mobile communication system, the PN sequence tracking step is performed in a finger, generally using either a delay lock loop (DLL) scheme or a tau-dither loop (TDL) scheme.
Both schemes designate a PN sequence's phase tracked in a previous PN sequence tracking step as a reference phase, and track an accurate phase of the PN sequence based on an energy difference between an early path preceding the reference phase by a predetermined phase and a late path behind the reference phase by the predetermined phase. The predetermined phase has a 1-chip length, or a ¼ or ½-chip length which is shorter than the 1-chip length. In an additive white Gaussian noise (AWGN) environment where no fading exists on a transmission channel, an energy difference between the early path and the late path is calculated by
                              E          ⁡                      (                          Δ              ⁢                                                          ⁢                              ξ                ⁡                                  (                  τ                  )                                                      )                          =                              N            1            2                    ⁢                                    E              c                        ⁡                          [                                                                    R                    2                                    ⁡                                      (                                                                  τ                        -                        Δ                                                                    T                        c                                                              )                                                  -                                  R                  ⁡                                      (                                                                  τ                        +                        Δ                                                                    T                        c                                                              )                                                              ]                                                          (        1        )            
In this equation, N1 represents the number of chips during which the early and the late energy are measured, Ec represents chip energy, Tc represents a chip duration, and R(·) represents an autocorrelation function of an impulse response of a pulse shaping filter. In addition, τ is a value representing a relative time difference between a reference phase and a PN sequence's phase of a received pilot signal. Specifically, τ is given subtracting a PN sequence's phase of the received pilot signal from an acquired PN sequence's phase. In addition, Δ represents an incremental a phase by which the early path precedes the reference phase or the late path is behind the reference phase. Therefore, ‘τ−Δ’ represents a phase of the early p, and ‘τ+Δ’ represents a phase of the late path. Herein, compared to the early path and the late path, a path of the reference phase is defined as an “on-time path.”
Meanwhile, the DLL scheme and the TDL scheme are different from each other in terms of the time instance at which the energies of the early path and the late path are being measured. The DLL scheme simultaneously calculates energies of the early path and the late path, whereas the TDL scheme sequentially calculates energies of the early path and the late path. The TDL scheme is simpler than the DLL scheme in hardware while providing a margin degradation in performance, thus contributing to power reduction. Therefore, the TDL scheme is preferred.
FIG. 2 is a block diagram illustrating an example of a PN sequence phase tracking apparatus employing the conventional TDL scheme (hereinafter referred to as “TDL PN sequence phase tracking apparatus”). Specifically, FIG. 2 illustrates a structure of a part for calculating an energy difference between an early path and a late path in accordance with Equation (1) when a burst pilot signal of FIG. 1 is provided. With reference to FIGS. 1 and 2, a detailed description will now be made of the conventional TDL scheme.
The PN sequence phase tracking apparatus employing the TDL scheme comprises of a first energy measurer 10 and a second energy measurer 20. The first energy measurer 10 sequentially calculates energies of an early path and a late path, and the second energy measurer 20 calculates a difference between the energy values of the early path and the late path, sequentially calculated in the first energy measurer 10.
After being converted into a baseband complex signal r,a pilot signal received from a base station is applied to a complex PN despreader 201. The baseband complex signal r is subjected to complex PN despreading in the complex PN despreader 201 by a PN sequence generated in any one of a first local PN sequence generator 205 and a second local PN sequence generator 207. The first and second local PN sequence generators 205 and 207, respectively, generate PN sequences of which phase precedes and are behind a reference phase by a predetermined phase. If it is assumed that a reference phase is τ and the predetermined phase is Δ, the first local PN sequence generator 205 generates a PN sequence having a phase ‘τ−Δ’, whereas the second local PN sequence generator 207 generates a PN sequence having a phase ‘τ+Δ’.
Based on the value of a function s(t), a switch 203 selects any one of the first local PN sequence generator 205 and the second local PN sequence generator 207 so that energies of the early path and the late path are alternately measured every pilot . For s(t)=+1, the first local PN sequence generator 205 is selected to calculate an energy value of the early path. For s(t)=−1, the second local PN sequence generator 207 is selected to calculate an energy of the late path.
The complex signal r despread by the local PN sequence in the complex PN despreader 201 is separated into an in-phase component and a quadrature-phase component and then provided to a first accumulation averager 209 and a second accumulation averager 211, respectively. Outputs of the first and second accumulation averagers 209 and 211 are provided to first and second squarers 213 and 215, respectively. Signals output from the first and second squarers 213 and 215 are summed up in a summer 217. An output of the summer 217 corresponds to an energy value of the early path or the late path according to a value of the s(t).
The output of the summer 217 is provided to a latch 219 and a subtractor 221. The latch 219 serves as a delay for generating a time delay corresponding to (N1+N2)-chip duration illustrated in FIG. 1. The subtractor 221 calculates a difference between the output of the summer 217 and the output of the latch 219, and provides its output to a multiplier 223. The multiplier 223 multiplies an output of the subtractor 221 by the function s(t). An output of the multiplier 223 corresponds to an energy difference between the early path and the late path.
However, in case the channel gain fluctuates due to fading the TDL scheme cannot track accurately an actual energy difference between the early path and the late path. This is because the channel gain when the energy of the early (or late) path is being measured may differ from that when the late (or early) path is being measured and, therefore, the same coefficient cannot be applied for both terms on the right hand side of Equation (1). That is, in case the channel gain fluctuates, the energy difference between the early path and the late path is not given by Equation (1) any further.
However, energies of an early path and a late path are simultaneously measured in the DLL scheme. Therefore, the DLL scheme is superior to the TDL scheme in performance. However, since the DLL scheme must include a separate device for measuring energies of the early path and the late path, its hardware complexity and power consumption are relatively high compared to those of the TDL.
Meanwhile, the performance of the TDL scheme tends to deteriorate as the time interval between pilot bursts increases. This is because the energy difference between the early path and the late path cannot be measured exactly due to the time-varying channel gain. In the case of the TDL, accordingly, an apparatus and method for combating the effect of channel fading are highly required.